Systems and devices for improving structural response to earthquakes are based on the principle of seismic isolation, in which energy is generally dissipated by mechanical dissipating devices. In order to prevent damage to maintain structural components, large horizontal displacements must be accommodated. For example, passive systems have been used for this purpose, including devices having lead cores within lead-rubber bearings, frictional sliding bearings, and other supplemental mechanical energy-dissipating devices such as steel, viscous, or visco-elastic dampers.
The use of active-control structures that attenuate excessive structural movement by hydraulic actuators are also known. The force exerted by the actuator is calculated in real-time using a control algorithm and feedback from sensors. Although this approach has shown to be effective, its applications are limited due to its high-power and continuous feedback signal requirements. Consequently, a considerable amount of recent research has focused on the use semi-active control strategies, which combine features of both passive and active control systems.
A vibration isolation concept, which relies on a spring arrangement with a non-linear stiffness that provides zero, or very small stiffness for a limited range of movement is known in the art. A “true” negative stiffness means that a force is introduced to assist motion, not oppose it. Negative stiffness devices have been applied to the development of vibration isolation systems for small, highly sensitive equipment (e.g., U.S. Pat. No. 6,676,101) and to seats in automobiles (Lee, C. M., Goverdovskiy, V. N. and Temnikov, A. I., “Design of springs with negative stiffness to improved vehicle driver vibration isolation”, Journal of Sound and Vibration, 302 (4), p. 865-874 (2007)). To date, however, this technology has been restricted to small mass applications because of the requirement for large forces to develop the necessary low or negative stiffness. The preload forces necessary to achieve negative stiffness are typically of the order of the weight being isolated. Thus, the application of negative-stiffness to a massive structure, like buildings and bridges, would require a spring force on the order of the weight of the massive structure. Such large spring forces would provide forces that would be physically very difficult and economically prohibitive to contain.
Negative stiffness concepts have been applied to isolating structures, but the concepts advanced have drawbacks. One concept advanced is a pseudo negative stiffness system where active or semi-active hydraulic devices are used to produce negative stiffness. However, such systems are complicated, and require high-power and continuous feedback in order to drive the active or semi-active hydraulic devices. Another example is a system in which a structure is placed on top of convex pendulum bearings. In this system, negative stiffness is generated due to the structure's vertical loads applied on the convex surface while elastomeric bearings placed in parallel provide positive stiffness. However, this system generates low effective stiffness that emulates the behavior of friction pendulum bearings. Complications of this system may arise due to the fact that the vertical loads are transferred through an unstable system, which generates constant negative stiffness for all displacement amplitudes.